Re: [PATCH v4] coresight: tmc-etr: Speed up for bounce buffer in flat mode
From: Suzuki K Poulose
Date: Tue Sep 14 2021 - 02:00:40 EST
On 13/09/2021 18:56, Mathieu Poirier wrote:
On Sun, Sep 05, 2021 at 11:21:44AM +0800, Leo Yan wrote:
The AUX bounce buffer is allocated with API dma_alloc_coherent(), in the
low level's architecture code, e.g. for Arm64, it maps the memory with
the attribution "Normal non-cacheable"; this can be concluded from the
definition for pgprot_dmacoherent() in arch/arm64/include/asm/pgtable.h.
Later when access the AUX bounce buffer, since the memory mapping is
non-cacheable, it's low efficiency due to every load instruction must
reach out DRAM.
This patch changes to allocate pages with dma_alloc_noncoherent(), the
driver can access the memory via cacheable mapping; therefore, load
instructions can fetch data from cache lines rather than always read
data from DRAM, the driver can boost memory performance. After using
the cacheable mapping, the driver uses dma_sync_single_for_cpu() to
invalidate cacheline prior to read bounce buffer so can avoid read stale
trace data.
By measurement the duration for function tmc_update_etr_buffer() with
ftrace function_graph tracer, it shows the performance significant
improvement for copying 4MiB data from bounce buffer:
# echo tmc_etr_get_data_flat_buf > set_graph_notrace // avoid noise
# echo tmc_update_etr_buffer > set_graph_function
# echo function_graph > current_tracer
before:
# CPU DURATION FUNCTION CALLS
# | | | | | | |
2) | tmc_update_etr_buffer() {
...
2) # 8148.320 us | }
after:
# CPU DURATION FUNCTION CALLS
# | | | | | | |
2) | tmc_update_etr_buffer() {
...
2) # 2525.420 us | }
Signed-off-by: Leo Yan <leo.yan@xxxxxxxxxx>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
---
Changes from v3:
Refined change to use dma_alloc_noncoherent()/dma_free_noncoherent()
(Robin Murphy);
Retested functionality and performance on Juno-r2 board.
Changes from v2:
Sync the entire buffer in one go when the tracing is wrap around
(Suzuki);
Add Suzuki's review tage.
.../hwtracing/coresight/coresight-tmc-etr.c | 26 ++++++++++++++++---
1 file changed, 22 insertions(+), 4 deletions(-)
diff --git a/drivers/hwtracing/coresight/coresight-tmc-etr.c b/drivers/hwtracing/coresight/coresight-tmc-etr.c
index acdb59e0e661..a049b525a274 100644
--- a/drivers/hwtracing/coresight/coresight-tmc-etr.c
+++ b/drivers/hwtracing/coresight/coresight-tmc-etr.c
@@ -609,8 +609,9 @@ static int tmc_etr_alloc_flat_buf(struct tmc_drvdata *drvdata,
if (!flat_buf)
return -ENOMEM;
- flat_buf->vaddr = dma_alloc_coherent(real_dev, etr_buf->size,
- &flat_buf->daddr, GFP_KERNEL);
+ flat_buf->vaddr = dma_alloc_noncoherent(real_dev, etr_buf->size,
+ &flat_buf->daddr,
+ DMA_FROM_DEVICE, GFP_KERNEL);
Suzuki and Robin - are you guys good with this new revision?
Yes, fine by me.
Suzuki